The present invention relates generally to clutch release bearings, also known as throw-out bearings, of the type particularly adapted for use with automotive clutches.
Such a clutch release bearing generally comprises a drive or thrust member which is adapted to coact with the release members of a clutch, in particular the fingers of the diaphragm spring in the case of a diaphragm friction clutch, an operating member whch is adapted to cooperate with an actuating member, in practice, a throw-out fork, and means for securing the drive or thrust member axially to the operating member.
The present invention relates more particularly to such clutch release bearings whcih are intended to be slidably received on a guide member which in the case of an automotive vehicle is a guide tube surrounding the input shaft of the transmission and fixed to the housing of the transmission. In such a case the operating member comprises a sleeve which is adapted to be entirely axially received on such a guide tube.
In the course of service the guide member inevitably accumulates dust, dirt and grit. The gradual buildup of such dirt may interfere with the proper sliding of the clutch release bearing necessary for operation and may even bind on the guide member.
In the case of a so-called self-centering release bearing, which maintains its self-centered position once it is obtained, the drive member is mounted for limited transverse movement in all directions with respect to the operating member, the sleeve of the operating member must be received with a relatively close fit on the guide member.
In my U.S. Pat. No. 4,261,625, assigned to the assignee of the present application, there is provided for meeting the apparently contradictory requirements of smooth sliding movement and a close fit a sleeve having along its inner surface radially inward projections or bosses adapted to slidingly engage the guide member and defining between one another channels or passages. In practice such projections or bosses are axially elongate splines and the passages or channels are axially elongate grooves therebetween.
Taking advantage of the self-lubricating properties of at least some plastic synthetic materials and the reduction of the frictional contact surface resulting from the splines, it was proposed in my aforementioned U.S. patent to provide dry contact, without any lubricant, between the sleeve of the clutch release bearing and the guide member on which it is received. The ends of the splines are axially outwardly tapering for channelling the dirt present on the guide member into the grooves thereby avoiding any undesirable contact between the sliding surfaces of the bosses or projections and the dirt or grit.
In practice such an arrangement has given satisfaction and its has the advantage of providing accurate location of the clutch release bearing with respect to the guide member, the splines on the sleeve of the operating member having the desired geometrical configuration for this purpose. Indeed, the dummy splines formed on the core of the mold for molding such an operating member may be subsequently individually trued.
For some applications, however, it is imperative to provide a grease reservoir between the clutch release bearing and the associated guide member.
Various arrangements have been proposed for this purpose. For example, in French patent publication No. 2,145,977 the sleeve of the operating member is provided with two radially inwardly resiliently deformable rings disposed at the respective ends of the axial bore in specially designed grooves in the inner surface of the bore, the rings having ridges engaging the guide member and defining the sought after grease reservoir or chamber therebetween. Owing to the resilient clamping action exerted by the rings the latter are detrimental to good sliding movement of the release bearing on the guide member. Further, such an arrangement necessitates the machining of annular grooves in the inner wall of the bore through the sleeve, and their mounting in the grooves is a rather delicate operation. Moreover, such an arrangement is not satisfactory in operation since the ridge profile of the rings may cram dirt and grit from the guide member.
In French patent publication No. 2,230,235 the sleeve of the operating member is made of flexible material and comprises at its axial ends annular beads which are in resilient engagement with the associated guide member and define, as above, a grease reservoir chamber between each other. But as in the preceding French patent publication owing to the elastic gripping of the guide member the annular beads tend to interfere with good sliding movement of the clutch release bearing on the guide member. In fact, given the actual flexible nature of the bead material, the beads are not suitable for scraping off any dirt or grit which may be present on the guide member.
In French patent publication, the provision of the sought after grease chamber translates into an uncertain location of the operating member with respect to the guide member, the elastic beads of the operating member sleeve enable skewing of the operating member relative to the axis of the guide member.
Admittedly, such skewing may be beneficial in the case of non-self-centering release bearings which is precisely the reason it is provided on the release bearing of French patent publication No. 2,230,235. But for the reasons developed above such skewing is normally not appropriate for sustained self-centering release bearings which maintain their self-centered position once it is attained.
Indeed, for ensuring the satisfactory position of the release bearing on the guide member, it is desirable to provide on the operating member relatively rigid projections or in other words an internal sheath or portion which is made of relatively rigid material.
It may be contemplated for the sought after grease chamber to provide on the inner surface of the bore of the sleeve of such relatively rigid material a recess closed at its axial ends such as is the rule for such grease chambers as exemplified by French patent publications Nos. 2,145,977 and 2,230,235. But when such a sleeve is of molded plastic synthetic material which contributes to good sliding on the guide member the formation of such a recess gives rise to difficulties which in practice are insurmountable.
Indeed, owing to the sleeve being of relatively rigid material it is impossible to unmold the sleeve elastically. Unmolding therefore would require the utilization of radially retractable cores making the cost of molding prohibitive.